1. Field of the Invention
The present invention relates to a method and system for the prevention of corrosion of conductive structures using semiconductor technology.
2. Discussion of the Background Art
A variety of methods for controlling corrosion have evolved over the past several centuries, with particular emphasis on methods to extend the life of metallic structures in corrosive environments. These methods typically include protective coatings which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel, and some nonferrous metals, such as aluminum, and to avoid the necessity for using more costly alloys. Thus, they both improve performance and reduce costs. However, such protective coatings typically have several pitfalls, including poor applicability to non-metallic structures that suffer from corrosion or fouling.
Protective coatings fall into two main categories. The largest of these categories is the topical coating such as a paint, that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium, that are designed to preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a primary purpose of mitigating and preventing corrosion. Each process is different: cathodic protection prevents corrosion by introducing an electrical current from external sources to counteract the normal electrical chemical corrosion reactions whereas coatings form a barrier to prevent the flow of corrosion current or electrons between the naturally occurring anodes and cathodes or within galvanic couples. Each of these processes provided limited success. Coatings by far represent the most wide-spread method of general corrosion prevention (see Leon et al U.S. Pat. No. 3,562,124 and Hayashi et al U.S. Pat. No. 4,219,358). Cathodic protection, however, has been used to protect hundreds of thousands of miles of pipe and acres of steel surfaces subject to buried or immersion conditions.
The technique of cathodic protection is used to reduce the corrosion of the metal surface by providing it with enough cathodic current to make its anodic dissolution rate become negligible (for examples, see Pryor, U.S. Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No. 4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No. 4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). The cathodic protection concept operates by extinguishing the potential difference between the local anodic and cathodic surfaces through the application of sufficient current to polarize the cathodes to the potential of the anodes. In other words, the effect of applying cathodic currents is to reduce the area that continues to act as an anode, rather than reduce the rate of corrosion of such remaining anodes. Complete protection is achieved when all of the anodes have been extinguished. From an electrochemical standpoint, this indicates that sufficient electrons have been supplied to the metal to be protected, so that any tendency for the metal to ionize or go into solution has been neutralized.
Recent work in the study of corrosion has found that electrochemical corrosion processes appear to be associated with random fluctuations in the electrical properties of electrochemical systems, such as cell current and electrode potential. These random fluctuations are known in the art as "noise". Researchers have begun to apply noise analysis techniques to study the processes of corrosion in electrochemical systems.
Riffe, U.S. Pat. No. 5,352,342 and Riffe, U.S. Pat. No. 5,009,757 disclose a zinc/zinc oxide based silicate coating that is used in combination with electronics in a corrosion prevention system. The zinc/zinc oxide particles in the coating are disclosed as having semiconductor properties, primarily a p-n junction at the Zn--ZnO phase boundary. When reverse biased, this p-n junction is described as behaving as a diode and inhibiting electron transfer across the boundary. This restriction limits electron transfer from sites of Zn oxidation to the sites of oxygen reduction on the ZnO surface. Effectively, there is increased resistance between the anode and cathode of local corrosion cells and corrosion is reduced.
On average, the Zn--ZnO based junction will be reversely biased due to the potentials associated with the oxidation of Zn at the Zn surface and the reduction of O.sub.2 at the ZnO surface. However, significant stochastic voltage fluctuations occur. These voltage fluctuations cause the junction to episodically become forward biased. When forward biased, electron transfer across the junction increases and there is an acceleration of the oxidation of Zn and reduction of O.sub.2. Effectively, there is a short circuit between the anode and cathode of local corrosion cells and corrosion is enhanced.
The Riffe patents disclose attachment of a fixed value capacitor in the electrochemical circuit of the corrosion prevention system. However, there is no way to control the level of capacitance nor any method suggested for determining the level of capacitance needed to effectively prevent corrosion in any given structure. Hence, it is necessary to use an overcapacitance in the system to be effective.